Multi-layer card attenuator for microwave frequencies



July 12, 1966 HBACHER ETAL 3,260,971

MULII-LAYER CARD ATTENUATOR FOR MICROWAVE FREQUENCIES Filed Dec. 5, 1964 2 Sheets-Sheet 1 FIG. I.

INVENTORS Helmuf Bacher on P.5e17f6r ATTORNEY July 12, 1966 H. BACHER ETAL 3,260,971

MULTI-LAYER CARD ATTENUATOR FOR MICROWAVE FREQUENCIES Filed Dec. 5, 1964 2 Sheets-Sheet 2 FIG. 7.

INVENTORS Helmuf Bocher Egon R.5eif1r ATTORNEY United States Patent 3 260,971 MULTI-LAYER CARD ATTENUATOR FOR MICROWAVE FREQUENCIES Helmut Bacher, Arlington, Va., and Egon R. Seitter, Silver Spring, Md., assignors to Weinschel Engineering Co., Inc., Gaithersburg, Md., a corporation of Delaware File Dec. 3, 1964, Ser. No. 415,742 13 Claims. (Cl. 333-81) This invention relates to attenuators for microwave systems and has for its primary object the provision of an improved attenuator which is substantially independent of frequency, from direct current up into the high microwave frequency range, including the P-band.

Patent No. 3,157,846 issued to Bruno O. Weinschel, for Card Attenuator for Microwave Frequencies, November 17, 1964, relates to an attenuator of the card type, i.e., one in which the resistive element is essentially a single resistive layer deposited on a two-dimensional surface. The present invention represents an improvement over that disclosed in said patent. The basic theory of the present application is essentially the same as in said patent. A card of resistive material in the form of a layer so thin that the field penetrates the entire layer, is connected to the ungrounded side of a microwave circuit by two electrodes as in the patent, and operated on the same general theory. The attenuator of the patent works well for moderate to high values of attenuation, but has been found to have some disadvantages for attenuators of low value, e.g., l0, 6, or 3 db, especially at the higher frequencies. It is desired that the attenuation should remain substantially constant from D.-C. to the highest frequency at which the attenuator is to be employed, e.g., 12,400 me, the upper end of the X-band. The prior attenuator, used with a parallel outer conductor arrangement, does not have a constant impedance along the attenuator, which means that match and frequency sensitivity of insertion loss will depend on frequency, If the attenuator is constructed as shown in the above patent, its overall length decreases with the attenuation value, e.g., a db attenuator has a much shorter linear length between electrodes than, for example, a 50 db attenuator.

At low attenuation values, it turns out that the attenuator becomes very short, and the linear distance between the two electrodes is reduced to the point where the field configuration is sufficiently disturbed by the mutual interaction of the two electrodes due to their proximity so that the design assumptions no longer hold true, especially at very high frequencies, due, in effect, to the capacitive interaction of the two electrodes. It is therefore desirable to lengthen the card attenuator for low attenuation values so as to physically separate the two electrodes by a sufficient distance so that they do not interact.

Another limitation of the prior card attenuator is that the power handling capabilities thereof are limited to current values which can be handled in the immediate vicinity of the high voltage electrode. This is due to the fact that most of the power dissipation is confined largely to the attenuator area close to the high voltage electrode (i.e., the electrode nearer the source). Consider, for example, the case of a 10 db attenuator where nine-tenths of the power supplied to the attenuator is dissipated in the attenuator, and most of this power is dissipated in the area close to the transition between the input electrode and the resistive film. Therefore, when the attenuator burns out it always burns out close to the junction of the high voltage electrode and the resistance film since the very thin resistance layer in this region could not dissipate that much power. A second objective of the present invention is to increase the power handling capability in this area.

In accordance with the present invention, the above and other difficulties are obviated by placing, in effect, a second layer of resistive material over a portion of the first layer of resistive material, and distributing said second layer in such fashion as to satisfy the transmission condition along the attenuator using an outer conductor (grounded) of constant cross sectional dimension, and also to decrease the total resistance, so that the electrodes can be further separated for the same value of attenuation.

Another limitation of the attenuation described in the above patent has been found to be a tendency to function as a wave guide at the higher frequencies, and it is also an object of the present invention to eliminate this effect, which is done by so shaping the attenuator unit with relation to the grounded outer conductor that wave-guide mode of propagation is not possible.

The specific nature of the invention as well as other objects and advantages thereof will clearly appear from a description of a preferred embodiment as shown in the accompanying drawings, in which:

FIG. 1 is a simplified schematic diagram of a fixed card attenuator, showing the principle of the invention;

FIG. 2 is a sectional View taken along line 22 of FIG. 1;

FIG. 3 is a longitudinal sectional view taken on line 3-3 of FIG. 4, of a practical embodiment of the invention;

FIG. 4 is a sectional view taken on line 44 of FIG. 3;

FIG. 5 is a detail view of the spring contact;

FIG. 6 is a View similar to FIG. 1 of an alternative attenuator card form;

FIG. 7 is a perspective view of an alternative form of attenuator element;

FIG. 8 is a sectional view taken on line 88 of FIG. 7;

FIG. 9 shows another modification of the basic attenuator principle;

FIG. 10 is a sectional View taken on line 1010 of FIG. 9;

FIG. 11 is a schematic equivalent circuit diagram used in explaining the principles of the invention; and

FIG. 12 shows another form of attenuator element.

In explaining the principles of the invention, it is helpful to look at the problem from the standpoint of classical transmission line theory. In the following, it will be shown that frequency sensitivity, VSWR, coupling between electrodes, and power handling can be optimized by finding arrangements which satisfy the known transmission line equations (see FIG. 11).

Z =characteristic impedance in ohms.

a: attenuation constant in nepers/ m. B=phase constant in degrees/m.

R=series resistance per meter in ohms/m. G=shunt conductance per meter in anhos/m. L-=series inductance per meter in henries/m. C=shunt capacitance per meter in farads/m.

The requirement for frequency independence 'of w and Z can be given by:

Substitute in II:

This consideration indicates that to meet the requirements, the following conditions must receive attention:

(1) The TEM mode has to be the mode of propagation.

(2) The shunt should be infinitely finely distributed in space around the whole circumference of the coaxial line three-dimensionally, and in this case there will be no inductive component in the shunt impedance.

(3) The line constants, R, G, L and C should not depend on frequency.

However, as we are interested in a unit which can be economical-1y produced, we can approximate the three conditions mentioned above and find a reasonable compromise. The single-plane attenuator can, of course, not meet condition No. 2 above, so its design must be such as to minimize this factor.

Another factor tending to disturb the desired linearity of the attenuator characteristic has been found to be the tendency of the card attenuator unit comprising the substrate material and its enclosing outer conductor to begin to function as a waveguide. This means that the TEM mode is not the only mode of propagation, and another waveguide mode which at higher frequencies tends to bypass the attenuator, is, of course, undesirable. This factor depends mainly on ground plane spacing and the attenuation per unit length ratio. It can, therefore, be minimized by reducing the ground plane spacing and the attenuation per unit length to the point where such waveguide action cannot occur within the frequency range for which the unit is designed. The characteristic impedance has to be maintained by changing the effective transmission line dimensions accordingly, as will be shown below.

The third factor is the power handling capability. It is mainly a peak power problem and depends mostly on the contact resistance between electrode and resistive film and on the resistance per area at the point where the imput electrode makes contact with the resistive film. As this resistance per area is considerably lower for the present invention, a substantial increase in peak handling power is obtained. The contact resistance between electrode and resistive film is minimized and stabilized by using a thin gold, silver, or other noble metal film with good adherence to the resistive film.

Referring to FIGS. 1 and 2, a thin sheet of rigid high grade insulating material 2 is positioned diametrically within a tubular outer conductive coaxial conductor element, and rigidly fastened thereto. The sheet or card 2 has uniformly distributed on at least one side thereof a very thin layer of resistance material 4, similar to that used in the prior attenuator. The longitudinal edges of the card and layer are covered at 6 with a strip of high conducting material such as a film of highly conductive noble metal, for the purpose of making good contact between the edge of the resistance layer and the outer conductor 3. The terminals 7 and 8 are connected to the centralconductor of the coaxial cable system with which the attenuator is to be employed, by suitable connectors, as will be shown in more detail below. The conductive layer 4 is deposited by known methods, as by spraying or painting a suspension of the conductive material in a suitable liquid carrier, or in a solvent in the case where a solution is employed, after which the unit is baked to remove the solvent or carrier, leaving a very thin layer of the conductive or resistive material. After layer 4 has been deposited, a second layer 9 is similarly deposited over the first layer, but does not extend entirely over the major portion of the surface as does the first layer, being instead suitably shaped, e.g., as shown in the figure, for a purpose which will be described in more detail below. Alternatively, the layer 9 can be deposited first, or the area of layer 9 can be coated (as by spraying through a mask) more heavily than the rest of the area. The essential thing is that the area corresponding to layer 9 be made thicker, so that this area has a lower unit resistance than the other layer, i.e., the remaining resistive portion. The shape of either area may be varied in accordance with the characteristics desired, in accordance with the theory given above, where the layer 9 represents the series resistor R and the layer 4 acts as the shunt resistor G. After both layers are deposited, the area corresponding to the ends of the terminals 7 and 8 is covered with a layer of highly conducting material such as silver or gold, and the electrodes 7 and 8 are applied so as to provide as good a contact as possible between the electrodes and the two resistive layers 4 and 9. While at first glance, the resistor area shape shown in FIG. 1 may appear to disregard the feature of conforming to the equipotential line distribution as described in the above patent, in practice, this is not really the case, since the better conducting area 9 alters the current distribution in such a fashion that the edge of the more highly conducting area still approximates an equipotential line, and this feature is therefore still fairly well approximated in the present invention.

As explained above, the second layer 9, with lower resistance per area, provides in effect a parallel circuit to the first layer 4, and thus reduces the attenuation for a given spacing of the electrodes and a given size of the attenuator. This enables the two electrodes to be spaced sufficiently far apart, even for low attenuation values, so that there is no capacitive interaction between them, which is undesirable, because this varies with the frequency, and therefore disturbs the linearity of the attenuator. It will be noted that the shape of the second layer is such as to provide maximum contact with the electrodes, and thus to provide sufliciently low voltage drop in the area around the contacts, which is the area of the greatest voltage gradient, where the power failure usually occurs due to overheating. By shaping the second layer in this fashion, the voltage gradient between electrodes tends to be more uniform, thereby enabling the maximum amount of power to be handled for a given size of attenuator. It should be noted that in these attenuators, the current flow is not simply in a straight line between electrodes, but is largely shunted off by the outer conductor 3, which is in contact with the resistive layer along a very appreciable portion of its length, in order to achieve this result. The upper layer 9 therefore tends to distribute the current in a rather complex fashion, but in general it can be seen that by shaping it as shown, the overall current distribution in the attenuator tends to be more uniform than would otherwise be possible with a single layer.

One factor tending to disturb the desired linearity of the attenuator characteristic has been found to be the tendency of the attenuator unit comprising the abovedescribed attenuator card and its enclosing outer conductor 3 to begin to function as a waveguide at higher frequencies. This is due to the fact that the conducting or semi-conducting attenuator surface 4 and the semi-cylindrical surface bounding it which is constituted by the outer conductor 3, form in themselves a waveguide which at the higher frequencies tends to bypass the attenuator, which is, of course, undesirable. This difliculty is minimized according to the present invention by reducing the clearance between the attenuator and the grounded elements to the point where such Waveguide action cannot occur within the frequency range for which the unit is designed. This is illustrated in FIG. 3, which is a crosssection of the commercial unit designed for practical use. It will be seen that the card attenuator 1 is supported within the outer coaxial conductor 3 by two conductive pieces 11 and 12, which, in effect, form an extension of the outer conductor so as to leave only a very small and restricted space 13 at the center, in which the card attenuator rests. The dimensions of this space 13 are so small that it cannot function as a waveguide except at frequencies considerably higher than the upper limit (18.0 go.) of the frequency range for which the unit is designed.

As shown in FIGS. 3 and 4, a practical commercial attenuator unit will include conventional connectors 16 and 17 so that the unit may be used in any conventional coaxial conductor circuit. The two conductive elements 11 and 12 are provided with semi-cylindrical outer surfaces to conform to the inner surfaces of the outer coaxial conductor 3, of the unit, and on their opposing surfaces also provided with channels 18 for reception of a comb spring 19 which serves both to center the card attenuator and to provide good contact between its conductive rim 6 and the outer conductor. Spring 19 may be of any suitable shape; for example, as shown in FIG. 5, it is cut to provide a plurality of independent comb-like teeth so as to uniformly distribute the spring pressure along the length of the edge of the card, and thus provide uniform contact along the entire edge of the card.

FIG. 6 shows another form of attenuator in which the spacing between the electrodes 7 and 8' is increased by using strips of highly conductive material and 10'. These, being essentially two-dimensional and presenting only their edges to each other, thereby increase the interelectrode capacitance and therefore reduce its undesirable effect previously noted. Furthermore, the thin resistive layer 4 of FIG. 1 instead of being uniformly distributed over the major portion of the plane area, is concentrated along strips 4 so as to produce desired values of attenuation, the shapes shown'being only exemplary of one possible configuration which is actually used in a commercial attenuator.

FIGS. 8 and 9 show a modification, employing the same principle of thicker and thinner resistive layers, which more closely approximate the desired three-dimensional uniform distribution of the thin shunt-resistance layer by providing several such layers distributed around the central thicker layer. To achieve this, a ceramic spoked-wheel base element 25 is provided, and the thicker layer of resistance material is laid on the outer surface of the inner hub of the elongated wheel 25 as shown at 9" so that it covers the four exposed quadrants of the hub. If desired, one or more of these layers 9" may be omitted. The thinner layers 4" extend along the sides of the spokes, as shown, to the rim of the wheel, which is covered with a layer of highly conductive material 6" so as to make good contact with the outer coaxial conductor 3". A conducting sleeve 26 may also be employed to insure minimum resistance at the outer coaxial conductor, which is usually maintained at ground potential at all points.

FIGS. 9 and 10 show a modification in which the thicker layer 9a is distributed over the surface of a round central supporting element 27 such as a ceramic rod, while the thin layer 9b is on one or both sides of a ceramic panel 28 which is preferably integral with rod 27.

FIG. 12 shows a modification similar to that of FIG. 9, except that in this case the central element 9b is a very thin metal tube which serves as the central series attenuator element, corresponding to resistive layer 94: of FIGS. 9 and 10. The shunt attenuator element 4a is similar to element 4a of FIG. 10, being supported on insulating panel 28. Highly conducting strips 31 and 32 on the edges of panel 28' make good Contact between the shunt resistance layer 4a and the series resistor 9b and the outer coaxial conductor 3' respectively.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of our invention as defined in the appended claims.

We claim:

1. A microwave attenuator comprising (a) a coaxial connector having an outer conductive member and an inner conductive member coaxially spaced and insulated therefrom,

-(b) a thin layer of resistive material uniformly distributed over an area defining a surface,

(c) said surface area comprising two sub-areas of different unit resistivity, the sub-area of lower unit resistivity being in good conductive contact with said inner conductive member and the sub-area of higher resistivity being in good conductive contact with said outer conductive member.

2. The invention according to claim 1, said sub-area of lower unit conductivity lying on a locus defined by the surface of a cylinder.

3. A microwave attenuator comprising (a) a thin layer of resistive material uniformly distributed over an area defining a surface,

(b) said surface area being bounded by two opposed and spaced linear sides and two opposed and spaced linear ends extending between said sides,

(c) a coaxial connector having an outer conductive member and an inner conductive member coaxially spaced and insulated therefrom,

(d) said surface area comprising at least two sub-areas of different unit resistivity, the sub-area of lower unit resistivity being in good conductive contact with said inner conductive member, and the sub-area of higher unit conductivity being in good conductive contact with said outerconductive member.

4. A microwave attenuator comprising (a) an insulating member having at least one surface area,

(b) a thin layer of resistive material on said surface,

(c) a coaxial connector having an outer conductive member and an inner conductive member coaxially spaced and insulatedtherefrom,

(d) said surface area being bounded by two opposed and spaced linear sides and two opposed and spaced linear ends extending between said sides,

(e) said resistive material along at least one of said sides being in good conductive contact with said outer conductive member,

(f) said inner conductive member being in good conductive contact with the resistive material at an area spaced from said sides,

(g) and a second layer of resistive material on said first layer, said second layer being also in good conductive contact with said inner conductor and covering only a part of the area covered by said first layer.

5. A microwave attenuator comprising (a) an insulating member in the form of a sheet or card having at least one surface area,

(b) a thin layer of resistive material on said surface,

(c) a coaxial connector having an outer conductive member and an inner conductive member coaxially spaced and insulated therefrom,

(d) said surface area being bounded between two opposed and spaced linear sides and two opposed and spaced linear ends extending between said sides,

(c) said resistance material .along at least one of said sides being in good conductive contact along one side with said outer conductive member,

(f) said inner conductive member being in good conductive contact with the resistive material at an area adjacent one of said ends and spaced from said sides,

(g) and a second layer of resistive material on said first layer, said second layer being also in good conductive contact with said inner conductor and covering only a part of the area covered by said first layer.

6. The invention according to claim 5, said second layer being distributed so as to reduce the local resistance in the vicinity of said inner conductive member more than the resistance in the region further from said member and thus to make the current distribution over the attenuator surface more nearly uniform.

7. The invention according to claim 5, and a second coaxial connector having an outer conductive grounded member electrically connected to said first outer conductive grounded member, and having a second inner conductive current member spaced and insulated from said second outer conductive member, said second inner conductive member being in contact with said resistive material at a second restrictive area spaced from said first restrictive area and also spaced from said sides, and being also in contact with said second resistive layer.

8. The invention according to claim 7, said outer co axial conductive member being round in cross section to constitute with said resistive layer a waveguide section, and separate metallic conductor means extending inwardly from and in electrical contact with said outer member, toward said resistive layer, to reduce the distance between the resistive layer and the grounded side and thus increase the lower cut-off frequency of the unit considered as a wave guide.

9. The invention according to claim 8, said resistive surface area being further conductively coated with highly conducting material along the side length which is in electrical contact with the outer member, said metallic conductor means comprising two metal pieces, each being semi-cylindrical on one side to engage the inner surface of the outer conductor; said pieces providing between them two diametrically spaced channels for respectively receiving the said conductively coated side lengths of the insulating member, and being spaced from said surface at all points between said channels, and spring contact means in said channels for establishing good electrical contact between said conductively coated portion and said metal pieces.

10. A microwave attenuator comprising (a) a coaxial connector having an outer conductive member and an inner conductive member coaxially spaced and insulated therefrom,

(b) an insulating element comprising an outer cylindrical member and an inner cylindrical member concentric therewith and spaced therefrom, and at least one insulating panel having a surface extending radially between said two members and extending longitudinally in the direction of the axis of the cylindrical members,

(c) a thin layer of resistive material on the outer cylindrical surface of said inner cylindrical member and extending onto the surface of said panel to the iner surface of the outer cylindrical member,

(d) good conductive means supported by said inner surface in contact with said thin layer and with said outer conductive member of the coaxial connector,

(e) a second layer'of resistive material on the outer 8 cylindrical surface of said inner cylindrical member in contact with said first layer,

(f) good conductive means on said inner cylindrical member in contact with said layers and with the inner conductive member of the coaxial connector.

11. The invention according to claim 10, including a plurality of said insulating panels, and a thin layer of resistive material on each panel extending between and in contact with the resistive means on the inner cylindrical member and the good conductive means on the outer cylindrical member.

12. A microwave attenuator comprising (a) an insulating member having at least one plane surface,

(b) a thin layer of resistive material on said surface,

(0) a coaxial connector having an outer conductive grounded member and an inner conductive current member coaxially spaced and insulated from said outer member,

(d) an outer hollow tubular conductor electrically connected at one end to said outer conductive member, said insulating member lying within said hollow member with said resistive material in contact with the tubular conductor along a line of contact parallel to the central axis of the tubular member but spaced therefrom,

(c) said inner conductive member being in contact with said resistive material at a locus spaced from said line of contact whereby current can flow through said attenuator between the inner conductive member and the outer conductive member,

(f) the inner surface of the tubular member being so shaped in cross-section that the distance from the central axis to said line of contact is greater than the distance from the central axis to any other axial line on said inner surface, whereby the cut-off frequency is higher than for a cylindrical hollow member enclosing a resistive member of the same dimensions.

13. The invention according to claim 12, and a second coaxial cable connector having an outer conductive grounded member electrically connected to said first outer conductive grounded member, and having a second inner conductive current member spaced and insulated from said second outer conductive member, said second inner conductive member being in contact with said resistive material at a second locus axially spaced from said first locus.

No references cited.

HERMAN KARL SAALBACH, Primary Examiner.

R. F. HUNT, Assistant Examiner. 

1. A MICROWAVE ATTENUATOR COMPRISING (A) A COAXIAL CONNECTOR HAVING AN OUTER CONDUCTIVE MEMBER AND AN INNER CONDUCTIVE MEMBER COAXIALLY SPACED AND INSULATED THEREFROM, (B:) A THIN LAYER OF RESISTIVE MATERIAL UNIFORMLY DISTRIBUTED OVER AN AREA DEFINING A SURFACE, (C) SAID SURFACE AREA COMPRISING TWO SUB-AREAS OF DIFFERENT UNIT RESISTIVEITY, THE SUB-AREA OF LOWER UNIT RESISTIVITY BEING IN GOOD CONDUCTIVE CONTACT WITH SAID INNER CONDUCTIVE MEMBER AND THE SUB-AREA OF HIGHER RESISTIVIETY BEING IN GOOD CONDUCTIVE CONTACT WITH SAID OUTER CONDUCTIVE MEMBER.
 12. A MICROWAVE ATTENUATOR COMPRISING (A) AN INSULATING MEMBER HAVING AT LEAST ONE PLANE SURFACE, (B) A THIN LAYER OF RESISTIVE MATERIAL ON SAID SURFACE (C) A COAXIAL CONNECTOR HAVING AN OUTER CONDUCTIVE GROUNDED MEMBER AND AN INNER CONDUCTIVE CURRENT MEMBER COAXIALLY SPACED AND INSULATED FROM SAID OUTER MEMBER, (D) AN OUTER HOLLOW TUBULAR CONDUCTOR ELECTRICALLY CONNECTED AT ONE END TO SAID OUTER CONDUCTIVE MEMBER, SAID INSULATING MEMBER LYING WITHIN SAID HOLLOW MEMBER WITH SAID RESISTIVE MATERIAL IN CONTACT WITH THE TUBULAR CONDUCTOR ALONG A LINE OF CONTACT PARALLEL TO THE CENTRAL AXIS OF THE TUBULAR MEMBER BUT SPACED THEREFROM, (E) SAID INNER CONDUCTIVE BEING IN CONTACT WITH SAID RESISTIVE MATERIAL AT A LOCUS SPACED FROM SAID LINE OF CONTACT WHEREBY CURRENT CAN FLOW THROUGH SAID ATTENUATOR BETWEEN THE INNER CONDUCTIVE MEMBER AND THE OUTER CONDUCTIVE MEMBER, (F) THE INNER SURFACE OF THE TUBULAR MEMBER BEING SO SHAPED IN CROSS-SECTION THAT THE DISTANCE FROM THE CENTRAL AXIS TO SAID LINE OF CONTACT IS GREATER THAN THE DISTANCE FROM THE CENTRAL AXIS TO ANY OTHER AXIS LINE ON SAID INNER SURFACE, WHEREBY THE CUT-OFF FREQUENCY IS HIGHER THAN FOR A CYLINDRICAL HOLLOW MEMBER ENCLOSING A RESISTIVE MEMBER OF THE SAME DIMENSIONS. 